1. Field of the Invention
The present invention relates to an electrodeposited copper foil laminated on an insulation substrate for printed circuit boards and a surface treatment method thereof. More particularly it relates to an electrodeposited copper foil for printed circuit boards and a surface treatment method in which a barrier layer of alloy of Zn—Co—As is formed on the electrodeposited copper foil so that adhesive strength to the insulation substrate, heat-resistance, and anticorrosion are improved, and in which innoxiousness and anticorrosion of equipment can be assured by using pyrophosphoric acid potassium as a complexing agent in the electrolytic process.
2. Description of the Related Art
Generally, printed circuit boards are widely used in a variety of electronic applications, such as radios, televisions, washing machines, VCR, computers, etc., and precise controllers for industrial electronic devices. Recently, printed circuit boards are developed to meet the demand for fineness, miniaturization and high density of integration of the electronic articles, which requires high quality of the printed circuit board.
For example, a flame-retardant prepreg that is made of epoxy resin and glass fibers impregnated therein as insulation substrates for industrial printed circuits. The printed circuit board is fabricated by laminating the electrodeposited copper foil on the insulation substrate at high pressure and temperature, and then etching the copper to the circuit pattern.
Raw materials for the electrodeposited copper foil are formed by consecutive electrolytic deposition in copper sulfate solution. The surface of the raw foil is nodularized for better adhesion with respect to the insulation substrate. Consequently, the electrodeposited copper foil is obtained by electrodepositing a barrier layer on the nodularized surface, and then treating the barrier layer with electrolytic chromate anticorrosion process.
However, brown stains and spottings frequently appear through out the resinous layer of the finished printed circuit board, which tend to adversely affect the insulation properties of the resin and consequently the performance of the printed circuits so that the physical properties of the final product are undesirable. The actual mechanism for the brown stain is not fully understood. However, the cause appears to be the result of chemical and mechanical interaction between the copper foil and the resinous layer. Such a drawback may cause degradation of the adhesion between the copper foil and the insulation substrate in the process of lamination that involves high pressure and high temperature treatment.
To solve the above problem, U.S. Pat. Nos. 3,585,010 and 4,049,481 disclose surface treatment methods of copper foil by which the brown stains are removed during the adhesion of the insulation substrate and the copper foil. A copper foil having excellent adhesion can be obtained. Also, Korean Pat. No. 065333 issued at 1993 provides a method in which alloy of Cu—As is coated using pulse current to thereby improve the adhesion to the insulation substrate.
According to the conventional methods to obtain the electrodeposited copper foil, copper oxides on the surface of the raw foil (that is laminated with the insulation substrate) are removed to activate the surface. Then, the surface is treated to provide a nodularized surface with copper, and a barrier layer is electrolytically formed on the surface. Finally, the surface is processed by anticorrosive treatment. A metal used for the barrier layer comprises nickel, tin, zinc, cadmium, arsenic or an alloy thereof. It is known that the above metal and alloy prevent stains at the interface of the adhesion, enable heat-resistant adhesion between the insulation substrate resin and the copper foil, and facilitate etching of the copper foil. Most electrolytic precipitation of metal is conducted in the solution of metal complexing compound rather than plain metal ions. Also, the electrolytic deposition formed in the bath of metal complexing compound exhibits excellent electrodeposition property so that uniform electrodeposition can be made to deep grooves as well as nodules on the surface to become minute, uniform and brilliant deposited film. Further the electrolytic deposition enables electrochemical coating with alloy of base metal and noble metal.
However, the product formed by the conventional method has inferior properties in heat-resistance and anticorrosion, particularly, in the process of copper clad laminating and the manufacture of the printed circuit board. Also, the surface treatment of the barrier layer using brass may cause ‘undercutting’ phenomenon in which the adhesive area between the copper foil circuit and the insulation substrate layer decreases because zinc has a rapid corrosion tendency compared to copper in the solution of copper chloride (I), iron chloride (II) or ammonium persulfate during the formation of circuits, which leads to the separation of copper foil circuit from the insulation substrate.
In the above conventional method, the complexing agent in the electrolytic solution to form the alloy layer during the formation of barrier layer comprises cyanogen CN, sodium citrate Na3C6H5O7, and tartaric acid.
However, the electrolytic solution containing cyanogen as the complexing agent is excellent in plating, but is a toxic solution that may produce poisonous gases during the manufacture and cause environmental pollution. Further, the solution containing sodium citrate is acidic resulting in corrosion of equipment, and tartaric acid is unstable particularly in the plating process of alloy metal having high ionization tendency.